The advent of portable electronics and low-voltage digital circuitry has resulted in a need for improved DC-DC converters. DC-DC converters that can provide a low voltage output (<2 V) regulated at high bandwidth, while drawing energy from a higher, wide-ranging input voltage (e.g., typically about a 2:1 range) are particularly useful for supplying battery-powered portable electronics. The size, cost, and performance advantages of integration make it desirable to integrate as much of the DC-DC converter as possible, including control circuits, power switches, and even passive components. Moreover, it is often desirable, if possible, to integrate the power converter or portions thereof with the load electronics.
One common approach is the use of a switched-mode power converter in which energy is transferred from the converter input to output with the help of intermediate energy storage in the magnetic field of an inductor or transformer. Such magnetics based designs include synchronous buck converters, interleaved synchronous buck converters, and three-level buck converters. Designs of this type can efficiently provide a regulated output from a variable input voltage with high-bandwidth control of the output.
For magnetics-based designs operating at low, narrow-range input voltages, it is possible to achieve extremely high switching frequencies (up to hundreds of Megahertz), along with correspondingly high control bandwidths and small passive components (e.g., inductors and capacitors). It also becomes possible to integrate portions of the converter with a microprocessor load in some cases. These opportunities arise from the ability to use fast, low-voltage, process-compatible transistors in the power converter. At higher input voltages and wider input voltage ranges, much lower switching frequencies (on the order of a few MHz and below) are the norm, due to the need to use slow extended-voltage transistors (on die) or discrete high-voltage transistors. This results in much lower control bandwidth, and large, bulky passive components (especially magnetics) which are not suitable for integration or co-packaging with the devices.
Another conversion approach that has received a lot of attention for low-voltage electronics is the use of switched-capacitor (SC) based DC-DC converters. This family of converters is well-suited for integration and/or co-packaging passive components with semiconductor devices, because they do not require any magnetic devices (inductors or transformers). An SC circuit includes of a network of switches and capacitors, where the switches are turned on and off periodically to cycle the network through different topological states. Depending upon the topology of the network and the number of switches and capacitors, efficient step-up or step-down power conversion can be achieved at different conversion ratios. An example of a step-down SC topology is shown in FIG. 1, which has an ideal conversion ratio M=Vo/Vi=2. When switches SA1 and SA2 are closed, the capacitors are charged in series as illustrated in FIG. 1A, and when switches SB1 and SB2 are closed, the capacitors are discharged in parallel as illustrated in FIG. 1B.
SC DC-DC converters have been described in prior art literature for various conversion ratios and applications, and the technology has been commercialized. These types of converters have found widespread use in low-power battery-operated applications, thanks to their small physical size and excellent light-load operation.
There are, however, certain limitations of the switched-capacitor DC-DC converters that have prohibited their widespread use. Chief among these is the relatively poor output voltage regulation in the presence of varying input voltage or load. The efficiency of switched capacitor converters drops quickly as the conversion ratio moves away from the ideal (rational) ratio of a given topology and operating mode. In fact, in many topologies the output voltage can only be regulated for a narrow range of input voltages while maintaining an acceptable conversion. Another disadvantage of early SC converters is discontinuous input current which has been addressed in some prior art approaches. These new techniques, however, still suffer from the same degradation of efficiency with improved regulation as previous designs.
One means that has been used to partially address the limitations of switched-capacitor converters is to cascade a switched capacitor converter having a fixed step-down ratio with a linear regulator or with a low-frequency switching power converter having a wide input voltage range to provide efficient regulation of the output. Another approach that has been employed is to use a switched-capacitor topology that can provide efficient conversion for multiple specific conversion ratios (under different operating modes) and select the operating mode that gives the output voltage that is closest to the desired voltage for any given input voltage. None of these approaches, however, are entirely satisfactory in achieving the desired levels of performance and integration.
A challenge, then, is to achieve the small size and ease of integration often associated with SC-based power converters while maintaining the high-bandwidth output regulation and high efficiency over a wide input voltage range associated with magnetics-based designs.